There are a variety of applications which require measurements of motion of a device relative to a region of interest, such as a surface of a document or a surface region within a robotically controlled system. Imaging techniques are known for measuring the relative velocity and determining the relative positions of an imaging device and a region of interest. In one such technique, a human operator views structural features, as imaged by a photosensor array, and judges position and motion based upon movement of the structural features within the field of view of the photosensor array. If the arrangement of the imaging system and the viewed region is controlled, it is possible for the measurements of position and/or velocity to be automatically interpreted from the relationship of the individual signals provided by the elements of the photosensor array. This technique may be used to monitor movement of an optical mouse relative to a rectangular grid by sensing and counting grid lines passing through a field-of-view of an imaging arrangement fixed to the mouse. The technique may also be used for monitoring rotation of a shaft having image-detectable encoding, or may be used to control operation of a robotic device.
A related operation is described in U.S. Pat. No. 5,578,817 to Bidiville et al., which discloses a device for use with a computer mouse or trackball. A pattern of spots is formed on a ball and is illuminated with one or more light sources. An optical element, such as a lens or diffractive element, may be used to focus an image of the ball on a sensor array. The signals generated by the array are then acted upon by logic circuits to convert the movement of the ball to X and Y components for determining corresponding movement of a cursor on a video display. Motion detection is based on tracking the differences in spatial intensity of the spots. The patent describes an "edge" as a difference of intensity between two adjacent black and white pixels at the sensor array. The basic operation of the sensor array is to recognize the edges and track changes over time. The cursor of the video display is manipulated in response to changes in the positions of the edges. The process is not limited to use with spherical objects, since it can be applied to tracking movement along a planar surface having a pattern of spots.
A method of determining the distance that is traveled by a sheet of paper within a printer or document scanner is described in U.S. Pat. No. 5,149,980 to Ertel et al., which is assigned to the assignee of the present invention. A linear array of light sensors is directed at the paper to sense light issuing from the paper. This produces a reference array of light signal strengths. After the paper is moved in a given direction, a second sampling of light signal strengths is generated from the light sensors. A cross-correlation function compares the second sampling to the reference array of signal strengths so as to determine the distance that the paper has moved in the given direction during the time period between acquiring the reference array and the second sampling. The cross-correlation function can be repeatedly applied as subsequent samplings of light signal strengths are generated using the linear array of light sensors. The one-dimensional approach may be extended to determine the vector of two-dimensional movement by substituting a two-dimensional array of sensor elements for the linear array.
Employing correlations among samplings from a photosensor array for the purpose of measuring motion is also described in U.S. Pat. Nos. 5,686,720 to Tullis and 5,644,139 to Allen, Tullis et al. 5,578,813 to Allen, Tullis et al., and 5,729,008 to Blalock et al., all of which are assigned to the assignee of the present invention. In a document scanning application of the inventions, navigation information is acquired simultaneously with acquiring image information of a scanned document. The navigation information may be acquired by using one or more navigation sensors that image "inherent structural features," which are defined as features of an original that are characteristic of processes for forming the original and are independent of forming image data and/or systematic registration data on the original. For example, if the original is a paper product, the inherent structural features may be paper fibers. Correlations among frames of navigation information from a navigation sensor may be used to map the image information that is acquired simultaneously with the navigation information. Since the process is able to arrange the image information based upon a detectable scan path of the image and navigation sensors, a reliable document reproduction can be achieved in applications that allow unconstrained free-hand movement of the sensors relative to the document.
While the correlation methods operate well in applications in which measurements of motion or position are based on images of features that are easily distinguishable, further improvements are desired. Even when the features that are imaged for purposes of acquiring navigation information are uniquely arranged, there is a susceptibility to inaccuracy if the imaging system or the image correlation processing is not sufficiently sophisticated to detect differences in the arrangement of the features.
What is needed is a method that enhances the sensitivity, accuracy and precision of feature differentiation, where the differentiation is used as a basis for determining either or both of velocity and navigation of an image-acquiring device relative to a region of interest.